The International Alloy Designation System naming scheme for wrought alloys utilizes a four-digit number (“xxxx”) for categorizing aluminum alloys. The first digit indicates the major alloying elements, the second, if different from 0, indicates a variation of the alloy, and the third and fourth digits identify the specific alloy in the series. For example, in alloy 7075, the first digit (“7”) indicates the alloy is in the zinc series and the third and fourth digits indicate a specific alloy used in aerospace applications. As used herein, a single number followed by three letters x, i.e., “7xxx”, means all of the alloys within a specific series. Thus, 7xxx means all 7000 series alloys, and 2xxx means all 2000 series alloys.
Large spacecraft components, such as propellant tanks, are most commonly constructed out of a combination of aluminum 2xxx and aluminum 7xxx alloys, especially including aluminum 7075 alloy. 7075 alloy is strong, lightweight, and readily available, and it has positive cryogenic properties that make it attractive for use in low-temperature environments. 7075 alloy is not fusion-weldable, however, and so construction of components made of 7075 alloy historically required the use of mechanical fasteners. More recently, however, solid-state welding techniques, such as friction stir welding, have been developed as alternative assembly processes.
Friction stir welding of aluminum alloys is considered a cost-saving process because it does not require consumable materials, and because most welds, even in very thick material, can be completed with only one or two passes. Friction stir welding is also highly energy-efficient, providing further cost savings. Typically, large cylindrical spacecraft propellant tanks constructed by friction stir welding consist of about five panels, joined along abutting edges to form a cylinder, with a ring and a dome joined at each end of the cylinder to form the closed, sealed tank. Such components can be friction stir-welded without multiple passes, inter-run cleaning, back gouging, or spatter; in many cases the welding can be done automatically without human intervention and can eliminate the need for post-weld dressing. Examples of friction stir welding used in the manufacture of space vehicles include U.S. Pat. Nos. 7,866,532, 8,123,104, 8,132,708 and 8,141,764, each of which is incorporated herein by reference in its entirety.
Despite these advantages, friction stir welding of aluminum 7xxx alloys presents challenges, particularly in applications such as spacecraft. Friction stir welds (FSWs) of aluminum 7xxx alloys are metallurgically unstable in their as-welded condition and are sensitive to stress corrosion cracking (SCC). Post-weld artificial aging (PWAA) can improve resistance to SCC, but adds significant time, processing, and operational costs to the construction process, and can degrade the mechanical performance of the parent material. In one example, it is recommended that PWAA heat treatments comprise 100 hours of heating at 225 degrees Fahrenheit or, alternatively, 325 degrees Fahrenheit for 4 hours. See, e.g., “Evaluation of Post-Weld Heat Treatments to Restore the Corrosion Resistance of Friction Stir Welded Aluminum Alloy 7075-T73 vs. 7075-T6,” “Conclusions,” by Christian Widener, Jul. 5, 2006 (THERMEC '06—Session G4: Friction Stir Processing). In addition, PWAA requires an oven that is large enough to receive the entire welded component or heater strips or blankets covering the weldments, all of which may be costly, difficult, or even impossible to obtain in the case of a very large component such as a spacecraft propellant tank.
There is thus a need in the art for apparatuses, methods, and systems that provide stable, stress corrosion cracking-resistant friction stir welds of aluminum 7xxx alloys, without the need for expensive and time-consuming post-weld artificial aging. It is further advantageous for such apparatuses, methods, and systems to provide improved heating and stirring functions.